Liquid developer composition

ABSTRACT

A LIQUID DEVELOPER COMPOSITION FOR CONVERTING A LATENT ELECTROSTATIC IMAGE TO A VISIBLE IMAGE AND FORM FIXING THE VISIBLE IMAGE TO PRODUCE PERMANENT AND SMUDGE FREE COPIES COMPRISING A TONER CONCENTRATE FORMULATED TO CONTAIN A ROSIN AND PIGMENT MATERIAL IN A LIQUID SOLVENT WHICH MAY BE ADMIXED WITH A CARRIER VOMPRISING A RESINOUS BINDER DIPERSED IN AN ORGANIC SOLVENT.

United States Patent U.S. Cl. 25262.1 4 Claims ABSTRACT OF THE DISCLOSURE A liquid developer composition for converting a latent electrostatic image to a visible image and for fixing the visible image to produce permanent and smudge free copies comprising a toner concentrate formulated to contain a rosin and pigment material in a liquid solvent which may be admixed with a carrier comprising a resinous binder dispersed in an organic solvent.

This application is a continuation of U.S. application Ser. No. 818 filed Jan. 5, 1970, and now abandoned.

This invention relates to electrostatic printing and more particularly to an improved liquid developer composition for converting a latent electrostatic image to a visible image and for fixing the visible image to produce permanent and smudge-free copy.

Processes for forming a latent electrostatic image, existing as an electrostatic charge pattern in a layer of material having high resistance, and for subsequently converting the latent electrostatic image into a visual pattern, are well known.

In one such process, an optical image is produced directly on a film or coating of a photoconductive pigment, such as zinc oxide dispersed in an insulating matrix, such as a synthetic resin, on the surface of a flexible carrier, such as a sheet of paper, metal or other electrically conductive backing. In this process, the photoelectrostatic coating is given an overall electrostatic charge while being protected from light and thereafter it is exposed to a light image of the subject to be reproduced. The electrostatic charge on the coating is dissipated in the areas struck by light and retained in the unexposed areas thereby to provide an electrostatic reproduction of the optical image. This latent electrostatic image is then converted to a visible image by a developing composition containing toner particles which are attracted to the latent electrostatic image in a positive toning process, or to the background area in a negative toning process.

In another process referred to as the Videograph process described in U.S. Pats. No. 2,996,573 and No. 3,075,- 859, a latent electrostatic image is inscribed by elements extending through a cathode ray tube directly onto the dielectric coating on the surface of a base sheet of paper or other highly electrically conductive material. The latent electrostatic image is retained on the dielectric coating and later developed into a visible image by the application of a suitable developing composition containing toner particles which are attracted to the latent electrostatic image or the background, as described above.

Liquid developer compositions for use with electrostatic images comprise a dispersion of a pigment or toner particles in a volatile liquid having a high dielectric strength and a high volume resistivity. The dispersed particles may carry either a positive charge or a negative electrical charge, depending on their chemical composition for either negative toning or positive toning respectively. The non-conductivity and the high dielectric strength of the volatile liquid in the liquid developing composition preserves the electrostatic image and permits the deposition of the dispersed toner particles to form a visible ima e. Liquid toners are described in the U.S. Metcalfe Pat. No.

"ice

2,907,674, the U.S. Straughan Pat. No. 2,899,335, the U.S. Mayer et a1. Pat. No. 2,390,911, the U.S. York Pat. No. 3,135,095 and the U.S. Dirks Pat. No. 3,155,546, and many others.

' Although liquid developing compositions represent an improvement over the use of earlier dry powder developers used for conversion of a latent electrostatic image to a visable image, none of the liquid developers are entirely satisfactory from the standpoint of producing a permanent visible image which is free of smear or blurring especially when rubbed. This stems from the inability of the toner particles to become permanently fixed on the surface of the copy sheet. Attempts have been made to fix the image by subsequent processing as by heating to fuse the toner particles, but such additional processing steps are insufficient and often incomplete.

It has been further attempted to formulate liquid developing compositions with a resinous or polymeric component dissolved therein as a binder, or with a liquid system containing a solvent which partially dissolves or softens a resinous component contained in the system or forming a part of the pigment particles for the purpose of bonding the pigment particles to the copy sheet. The presence of such dissolved or partially dispersed organic binder components brings about agglomeration of the toner particles with resulting separation in the developing composition and non-uniformity in the deposition of the toner particles for development of the resistible image.

Liquid developing compositions heretofore proposed which contain dissolved resinous or binder components have been found to dry on the applicator rollers to cause staining of the original copy, cobwebbing in the operation of the applicator rolls and sticking of the rolls. When the developing composition contains a solvent for the resinous component in sufiicient amount to retain the resin in solution in the developing composition, the composition is capable of attacking the insulating or dielectric layer to provide excessive background color.

It is accordingly an object of the present invention to provide a liquid developing composition for latent electrostatic images containing a resinous binder wherein the deposited toner particles become permanently fixed to the copy in order to provide good readable copy without the aforementioned disadvantages.

It is a further object of the present invention to provide a liquid developing composition which completely avoids agglomeration of the toner particles.

It is a further object of the invention to provide a liouid developing composition which is formulated to con ain a toner concentrate admixed with a carrier containin a resinous binder to fix the toner particles to the copy sheet.

It is yet another object of the present invention to provide a developing composition for latent electrostatic images wherein additional processing steps are not required to fix the toner particles deposited during development of the image; in which the ima e becomes fixed during normal drving of the copy; and, in which the developing composition is otherwise the same as com ositions heretofore employed from the standpoint of characteristics and use so as to enable application in a conventional manner without change in equipment, procedure and processing steps.

The concepts of the present invention reside in a liquid develo ing composition for latent electrostatic images which is formulated of 1) a toner concentrate containing a high concentration of pigment particles suspended in anorganic solvent by means of rosin, and (2) a compatible carrier system formulated of a binder capable of forming a dielectric film or coating which in part serves to anchor the igment particles to copies upon drying to thereby fix the image without the necessit to make use of heat. solvent or the like techniques. When combined to provide a developing composition, the foregoing materials constitute a stable developing composition in which no agglomeration of the toner particles is experienced so as to produce an image on a clean white background which is easy to manufacture and use.

As used herein, the term rosin is meant to refer to rosin and derivatives thereof. A wide variety of rosins may be employed in accordance with the practice of the present invention. Included are tall oils of rosin as well as their ester and hydrogenated ester derivatives. It has been found that best results are obtained by the use of Hercolyn D," a hydrogenated methyl ester of rosin, marketed by the Hercules Powder Co. This material serves at least in part to disperse the pigment or toner particles in the toner concentrate, in addition to service as a charge directing agent and a partial fixing agent to form a thin film to anchor the pigment particles to the copy sheet. The amount of rosin present in the toner concentrate is generally within the range of 40% by weight, and preferably 28-38% by weight. When the toner concentrate is combined with the compatible carrier to provide the liquid developing composition, the rosin generally constitutes between 0.02-.08% by wt. of the developing composition.

The toner concentrate is also formulated to contain a dispersant or restrainer in order to insure that the pigment or toner particles will remain in suspension in the toner concentrate and in the developing composition. As a result, the toner will not settle out thereby to avoid tailing and agglomeration, as heretofore experienced in the prior art. Metal stear-ate soaps have been used for this purpose in the past, but it has been found that remarkably superior results are obtained by the use of Fuel Oil Additive #2, marketed by Du Pont, which is a mixture of 50% by weight methacrylate polymer and 50% by weight kerosene. Without limiting the invention as to theory, it is believed that the eflic-acy of F.O.A. #2 stems from the fact that it serves as a partial fixing agent to anchor the toner particles to the copy sheet, and a charge director, While also rendering the developing composition more hydrophobic. The F.O.A. #2 or other restrainer is employed in the toner concentrate generally within the range of .1 to 10% by weight and preferably 3-7% by weight. When the toner concentrate is combined with the compatible carrier to provide the developing composition, the F.O.A. #2 present in the developing composition, will preferably be employed within the range of 0.0002 to 0.02% by Weight. It has been found that too much of the restrainer results in a loss of density in the developing composition.

The toner concentrate may also contain, if desired, a parafiinic or naphthenic mineral oil which operates to produce blacker copy, and also serves to minimize settling of the pigment particles in the toner concentrate. A wide variety of mineral oils have been found to be suit-able for this purpose, such as a paraffinic mineral oil, as represented by Citgo Oil 90102, or a n'aphthenic oil, as represented by A. B. Dick material number 30122 Diamond Paraffinic Oil. The mineral oil is generally employed in the toner concentrate in an amount up to about 10% by weight, while the amount present in the developing composition may range up to about 0.02% by weight.

The toner particles may be selected from a Wide variety of solid pigment particles which are electrically attractable. Suitable organic or inorganic materials are described in the Carlson Pat. 2,297,691, talcum powder, aluminum bronze, carbon dust and the like. It is generally preferred to make use of a powdered dyestuff, such as nigrosine, or a carbon material, such as carbon black, lampblack, channel black or the like. The toner particles may be in the form of pigment particles formulated of a suitable dyestuff or a carbon black embodied in a resinous carrier, as described in U.S. Pat. Nos. 2,907,674, 2,891,911, 3,135,- 095 and 2,899355.

In accordance with a preferred embodiment of the present invention, the toner particles are formulated to contain 4 one or more carbon blacks, such as Mogol A, Raven 11 or GAF permanent black toner No. 84-5001, which is admixed with a nigrosine dye, such as Nigrosine Base B or Nigrosine Base SSB and then treated with an organic polar solvent. It has been found that solvent treated pigment particles provide a better defined visible image with a cleaner background. A wide variety of polar solvents may be used for treating the pigment particles, including aliphatic ketones, such as acetone methyl ethyl ketone, diethyl ketone, etc; aliphatic alcohols, such as methanol, ethanol, isopropanol, etc., aliphatic aldehydes and the like. The amount of toner particles present in the toner concentrate is generally within the range of .01 to 10% by weight whereas the amount present in the liquid developing composition is generally quite low, such as within the range of 0.00002 to 0.02% by weight.

:It is frequently advantageous to include toning agents to improve the image color. As toning agents it is preferred to make use of a flushed blue color or alkali blue since they have been found to provide a cleaner background and denser images. Alkali Blue R powder 55- 4570, marketed by American Cyanamid, may be conveniently used for this purpose.

It is generally preferred to formulate the toner concentrate to include an aliphatic solvent in order to maintain the pigment or toner particles in dispersion. Aliphatic solvents used for this purpose preferably have a high volume resistancy in excess of 10 ohms cm. so as to avoid dissipation of the charge in the electrostatic image, serve to avoid attack on the binder in the photoconductive coating. Aliphatic solvents marketed by Humble Oil & Refining Co. have been found to be useful for this purpose and include Isopar H having a flash point of 123F., and Isopar G having a flash point of 104F. Both of these have a KB value of about 27, and low odor level.

The compatible carrier system, for use in admixture with the toner concentrate to provide the liquid developing composition, is a generally formulated to contain a binder component dispersed in a suitable aliphatic solvent which will not attack the binder component of the photoconductive coating. The binder component in the carrier system is generally a polymeric material which operates to provide a film or coating for the copy upon evaporation of the solvent to thereby anchor the pigment particles to the copy sheet.

While various binders, such as poly-vinyl acetate, polyvinyl acetate copolymers containing carboxyl groups, polyvinyl chloride, polyvinyl acetate copolymers, polystyrene, styrene-butadiene copolymers, alkyd and modified alkyd resins may be used for this purpose, it has been found that superior results may be obtained by the use of Pliolite VTL, which is a modified polyvinyl toluene resin marketed by Goodyear. This preferred binder is insoluble in aliphatic solvents, and hence it is generally preferred to dissolve the polyvinyl toluene in a small amount of aromatic solvent, such as Napoleon -A, marketed by the Deeprock Division of Kerr-McGee Company, and then disperse the dissolved resin in an aliphatic solvent, such as Isopar G, in order to provide a fine dispersion of the binder particles in the aliphatic solvent of the resin system. The binder may be employed in the developing composition in an amount within the range of .00001 to .05% by weight.

Both the carrier and the toner concentrate are completely compatible one with the other in any desirable ratio to provide the liquid developing composition so that the dye concentrate in the liquid developer composition can be easily increased or varied to raise the dye concentrate by addition of the toner concentrate, or to lower the concentrate by addition of the carrier to maintain the optimum dye concentrate. Such additions of concentrate or carrier can be effected without noticeable change in the viscosity of the developer liquid to provide high loading in the developer.

As a result, if the dye content is rapidly depleted during use of the developing composition by high speed developing of latent electrostatic images, the optimum concentration of the dyestufi in the liquid developer can be maintained by measured or substantially continuous additions of concentrate. By the same token, loss of diluent by reason of slow or low rate of development can be compensated for without an undesirable increase in the dye concentrate by additions of the carrier.

Having described the principal concepts of the present invention, reference is now made to the following examples, which are provided by way of illustration and not by way of limitation, of the invention.

EXAMPLE 1 Pigment or toner particles are solvent treated by dissolving Nigrosine Base B in 675 m1. of ethanol, to which 180 g. of GAF carbon black 845001 are added. The mixture is filtered, and the filtrate is saved. Then, the solids are washed with 675 ml. of methyl ethyl ketone, and the filtrate of this washing is combined with the first filtrate to provide a mixture, which is used to wash the solids again. Thereafter, the solids are dried and powdered.

The solvent treated toner particles are then formulated into the following composition.

Solvent treated toner 5.125 Toning agent (Alkali Blue R Powder 55-4570 from American Cyanamid) Hydrogenated methyl ester of rosin (Hercolyn Kerosine-methacrylate polymer (equal parts by weightF.O.A. #2) 4.721

The foregoing composition is then ground until a smooth paste is formed, which is then admixed with a paraflinic oil and an aliphatic solvent to provide the following toner concentrate.

G. Toner particles 5.1 Toning agent 0.3 Rosin ester 32.8 F.O.A. #2 4.7 Paratfin oil (Citgo Oil 90102) 7.0 Aliphatic solvent (Isopar H) 50.0

EXAMPLE 2 The carrier for the above toner concentrate may be formulated as follows:

A polyvinyl toluene (Pliolite VTL) solution is prepared by dissolving 1 gram of the polyvinyl toluene in 7 grams of an aromatic solvent. This solution is then formulated with an aliphatic solvent to provide the following carrier composition.

M1. 25 by weight solution of polyvinyl toluene 2 Aliphatic solvent (Isopar G) 1500 EXAMPLE 3 Pigment or toner particles are prepared by dissolving 60 g. of Nigrosine SSB in 675 ml. of ethyl alcohol, to which 180 g. of Raven 11 carbon black are added. The mixture is filtered, with the filtrate being saved; then the solids are washed with 675 ml. of methyl ethyl ketone. Thereafter, the filtrate from this washing is combined with 6 the first filtrate to provide a mixture which is used to wash the solids again. Thereafter, the solids are dried and powdered.

The solvent treated toner particles are then formulated into the following composition.

G. Solvent treated toner 5.125 Toning agent (Alkali Blue R Powder 55-4570) 0.500 Hydrogenated methyl ester of rosin 32.853 FDA. #2 3.54

The foregoing composition is then ground on a threeroll mill until a smooth paste is formed, which is then admixed with a paraflinic oil, an aliphatic solvent and The toner concentrate of Example 3 may then be ad mixed with the carrier to provide liquid developing composition containing between .01 to 3% by weight of the toner particles.

In use, the copy sheet containing the latent electrostatic image is wet with the liquid developing composition, with the immersion of the sheet in a bath of the developing composition or by flow-coating the composition over the image surface or by application of the liquid developing composition onto the image surface by a roller-coater. The copy sheet which is wet with the developing composition is then advanced through squeeze rolls to remove excess liquid. The toner particles are attracted to the latent electrostatic image for visual development of the image on the copy sheet. After development, the copy is allowed to dry by evaporation of the liquid, with or without the application of heat. The aliphatic solvents evaporate from the sheet to thereby reduce the binder and the rosin to a tackified or adhesive stage whereby the deposited toner particles become bonded to the underlying coating so as to become permanently fixed on the copy upon complete drying of the sheet.

It will be apparent that I have provided a new and improved liquid developing composition containing a resinous binder which is completely free from agglomeration of the toner particles so as to avoid the disadvantages of the prior art. The developing composition of the present invention provides well defined images on a clean white background which are more readable than those heretofore obtainable.

It will be understood that various changes may be made in the details of formulation without departing from the spirit of the invention, especially as defined in the following claims.

I claim:

1. A liquid developer composition for converting a latent electrostatic image to a visible image comprising a liquid aliphatic organic solvent having a resistivity in excess of 10 ohm-cm, 0.02 to 0.08% by weight of hydrogenated methyl ester of rosin, 0.00002 to 0.02% by weight of electrostatically attractable pigment particles being in the form of carbon black which has been admixed with a nigrosine dye and treated with an organic polar solvent prior to incorporation into the composition, 0.0002 to 0.02% by weight of a dispersant in the form of a methacrylate polymer to retain the pigment particles dispersed in the solvent and 0.00001 to 0.05% by weight of a binder capable of forming a dielectric film selected from the group consisting of polyvinyl toluene, polyvinyl acetate, vinyl acetate copolymers, polyvinyl chloride, polystyrene, styrene-butadiene copolymers and alkyd resins dispersed in the solvent.

2. A composition as defined in claim 1 which also includes a toning agent.

3. A composition as defined in claim 1 which also includes a mineral oil.

4. A composition as defined in claim 1 wherein the binder is a polyvinyl toluene.

References Cited UNITED STATES PATENTS NORMAN G. TORCHIN, Primary Examiner J. D. BRAMMER, Assistant Examiner 1 o I UNITED STATES PATENT OFFICE. CERTIFICATE OF CORRECTION PatentNo. 3.793.234 Dated Feb. 19, 1914 Inventor(s) Mary K. Ormsbee v It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 4, after "60305" please insert assignor to A. B. Dick" company, Chicago, Illinois Sighed and sealed this 3rd day of September 1974 (S Att'est:

M'cCOY M. GIBSON, JR. 0. MARSHALL DANN Attesting Officer Commissioner of Patents 

